1. Field of the Invention
The present invention relates to mechanically actuated switches and more specifically to mechanically actuated keypad switches using a conductive rubber membrane.
2. Description of the Prior Art
Keypad switches for controlling circuits included in printed circuit cards are known in the art. In particular, one type of switch consists of a key mounted over exposed traces of a printed circuit card. The key is supported over the exposed traces by an elastic membrane and contains shorting means on its underside which interconnect appropriate exposed traces when the key is depressed.
In many applications, several interconnections must be made simultaneously when the key is depressed. Consider, for instance the dual multipole switching circuit depicted in FIG. 1 wherein terminals 1, 2, and 3 must be interconnected at the same time that terminals 4, 5, and are interconnected.
A prior art keypad switch for perforiming this particular function is illustrated in FIGS. 2A and 2B. A key 7 is supported over a printed circuit card 8 by means of an elastic membrane 9. The portion of the printed circuit card underneath the key contains exposed traces which are to be interconnected when the key is depressed. Conducting discs 10 and 11 are mounted on the underside of the key so as to contact the appropriate exposed traces on the printed circuit card when the key is depressed.
Typically, the key 7 in such prior art switches is formed from relatively thick rubber and the supporting membrane 9 which encircles the base of the key is formed from thin sections of the same material. Typically, also, the conducting discs 10 and 11 are formed from conducting rubber pucks moulded onto the rubber key 7.
FIG. 2B illustrates how the electrical components of the prior art switch would be arranged to perform the dual multipole switching function indicated in FIG. 1.
The conducting rubber pucks 10 and 11 are arranged over the exposed trace arrays 12 and 13 on the printed circuit card.
Each of the exposed trace arrays 12 and 13 effectively consists of a plurality of conducting fingers, each connected to one of the three associated terminals and uniformly interspersed throughout the array. When the key is depressed, the rubber pucks short out several adjacent conducting fingers, thus interconnecting the associated terminals and effectively closing the corresponding switch as depicted in FIG. 1.
Although the above-described prior art switch has great utility, it does have some disadvantages.
Since the key must accommodate two conductive rubber pucks to cover a minimum area, the key must be a rectangle with a length more than two times the diameter of one puck and width at least equal to the diameter of one puck. Since such switches are frequently used in an environment where space is at a premium, such long keys can be a serious problem.
Additionally, the shape of the key permits it to be pressed in a manner to collapse only one end of the key membrane causing only one conductive rubber puck to contact the associated exposed trace array so that only one switch closes. Furthermore, even when both switches close, there is little guarantee that simultaneous contacts will be made.